This invention relates to a phase and frequency detector circuit, more particularly to a phase and frequency detector circuit used in a phase-locked loop in, for example, the local oscillator of a frequency converter.
Phase detectors are essential components of phase-locked loops, which are widely used for frequency control in electronic communication, navigation and computer systems. A phase detector responds to the difference in phase, or in frequency and phase, between an input signal and a reference signal and generates an output signal that varies depending on this difference. In a phase-locked loop, the phase detector output controls the frequency of a voltage-controlled oscillator, the output of which is fed back to the phase detector as an input signal. In this way the phase detector causes the voltage-controlled oscillator to lock onto the frequency of the reference signal. Alternatively, a divider, such as a programmable divider, can be inserted in the feedback loop to have the voltage-controlled oscillator to lock onto a desired multiple of the reference frequency.
Two well-known examples of prior-art phase detectors are the R-S flip-flop circuit and the exclusive-OR gate. Both the exclusive-OR gate and the R-S flip-flop, however, suffer from the disadvantage of being unable to discriminate frequency. A resulting problem in phase-locked loops is that the loop tends to lock to harmonics of the reference frequency unless an additional device such as a sweep oscillator circuit is provided to match the frequencies of the reference signal and input signal.
Prior-art phase detectors capable of discriminating frequency as well as phase have been configured using, for example, D flip-flops. A problem with such circuits, however, is that they sacrifice gain (with respect to phase). Accordingly, they are unsuitable for use in phase-locked loops that must operate at low levels of phase noise.